The human major histocompatibility complex harbors two sets of extensively polymorphic genes called class I and class II genes. The molecules derived from these genes have pivotal roles in the immune recognition of foreign antigens. Various allelic forms of the class II genes are associated with several diseases, most of which have an autoimmune etiology. A profound understanding of the physiological function of the class II molecules and their genes is required to understand their role in autoimmune diseases. The number and expression of class II genes with special regard to the DX, DX and DO loci will be examined. Efforts will be made to identify a DZ beta and a DO alpha genes, respectively. Co- expression of DX alpha and DO beta cDNA clones, respectively, with candidate DZ beta and DO alpha genes and cDNA clones may identify the missing genes. Since it may be expected that the DO molecule is non-polymorphic it may not serve as a restricting antigen-presenting element. The possibility that it is recognized by another type of receptor than the T-cell receptor will be explored by analyzing the binding of soluble DO molecules to appropriate cell types. The DX gene pair exhibits all features expected of functional genes but transcripts have not been identified. The possible existence of silencer sequences that prevent expression of the DX genes in the cell types examined will be explored by deletion mutations of intron sequences followed by transfections and measurements of transcript synthesis. The genetic polymorphism of the class II genes may be complemented by a phenotypic polymorphism generated by heterodimer formation of chains derived from different loci. The existence of such hybrid class II molecules will be examined in HeLa cells transfected with various combinations of expression vector-inserted cDNAs. The role of such molecules in syngeneic and allogenic MLR will also be studied with the aims to explore whether immunological tolerance exists to them and to the DX, DO and DZ molecules and whether immunological tolerance exists to them and to the DX, DO and DZ molecules and whether linear or conformational determinants are responsible for the MLR reactivity. A detailed understanding of the function of the class II molecules required knowledge about their three-dimensional structure. Soluble class II molecules suitable for such analyses will be generated by expressing mutated cDNA clones in a baculovirus expression system. The proposed studies will further our understanding of the expression of human class II molecules and may reveal important aspects of how the immune system recognizes these molecules in transplant rejection situations.